Limbic P3 potentials, seizure localization, and surgical pathology in temporal lobe epilepsy

Limbic P3 event-related potentials were recorded from mesial temporal electrodes implanted for presurgical investigation in 70 patients with intractable focal seizures. In 46 (81%) of 57 patients with unilateral temporal lobe epilepsy, the limbic P3 potential was absent or rudimentary ipsilateral to the seizure focus and a robust P3 potential was always elicited from the nonepileptogenic temporal lobe. Bilateral P3 potentials were recorded in 6 patients (10%) with unilateral temporal lobe epilepsy. In the remaining 5 patients in the group with unilateral temporal lobe epilepsy, results showed P3 bilaterally absent (2 patients), P3 present in a unilateral investigation (1 patient), P3 absent contralateral to the seizure focus (1 patient), and technically unsatisfactory recordings (1 patient). Bilaterally absent P3 potentials were noted in 2 patients with bilateral temporal lobe epilepsy. In 6 patients with technically adequate P3 studies and extratemporal seizures, bilaterally present P3 potentials were noted. Sensitivity and specificity of P3 absence as a predictor of an epileptogenic temporal lobe were 87% and 95%, respectively. Tissue specimens of the hippocampus were available in 22 patients (43%). Thirteen hippocampi showed sclerosis, all of which were associated with unilaterally absent P3 potentials. Nine hippocampi were normal (5 patients with the P3 absent, 4 with P3 present). Sensitivity and specificity of an absent limbic P3 as a function of hippocampal pathological findings were 100% and 44%, respectively. Absent limbic P3 potentials in temporal lobe epilepsy thus indicate structural or functional hippocampal abnormality and may add important information in presurgical evaluation with depth electrodes of patients who have temporal lobe epilepsy.